1. Field of the Invention
This invention relates to suspension polymerization of vinyl chloride, particularly to a process for preparing a vinyl chloride resin having a high porosity as well as good plasticizer take-up and capable of producing products with a very small number of fish eyes.
2. Description of the Prior Art
Polyvinyl chloride (PVC) is a useful resin having excellent physical properties, and a variety of PVC from rigid to soft types have been used.
Among them, for soft resins, various plasticizers are employed and therefore as one important characteristic PVC is required to have excellent plasticizer take-up. For improvement of plasticizer take-up, the particles of PVC must be made more porous.
There have been proposed a large number of methods for preparation of PVC excellent in plasticizer take-up in the prior art. For example, the method proposed in Japanese Pre-examination Patent Publication (KOKAI) No. 115890/1977 is a method for preparing a porous PVC, which comprises carrying out suspension polymerization of vinyl chloride in the presence of a partially saponified polyvinyl alcohol having a saponification degree of 37 to 70 mol % and a polymerization degree of 160 to 500.
The methods proposed in Japanese Pre-examination Patent Publication (KOKAI) Nos. 14607/1982 and 229907/1985 are intended to obtain a porous PVC by starting the polymerization with the use of a surfactant or a polyvinyl alcohol with a low saponification degree as a dispersing agent, and adding a water-soluble dispersing agent when a predetermined polymerization conversion is attained.
According to these methods, PVC which is porous and excellent in plasticizer take-up to some extent can be obtained, but scale is liable to deposit in the polymerization vessel, and also when the amount of the vinyl chloride monomer is increased or the polymerization time is shortened, there is involved the problem that the polymer obtained will become coarse particles, or become a mass. Thus, they are not necessarily practical methods with high production efficiency. This is because, when a surfactant or a polyvinyl alcohol with a low saponification degree is used from the initial stage of polymerization, the skin layer on the surface of the polymer particles will be lost, whereby the suspension system becomes unstable.